Astro에서 Timing이 민감한 cell들을 인접하게 place하거나 output buffer를 output port에 최대한 가깝게 place할 때 유용한 기능.
Thanks to Dr. Choi~

Donny’s Guideline: 직접 P&R을 할 줄 알면 더 좋지만 그렇지 않더라도 P&R툴의 기능을 알아야 back-end engineer에게 요구를 할 수 있으므로 유용한 기능들은 알아두는 것이 좋다. 또, Astro의 command까진 아니더라도, 합성에 사용한 synopsys의 constraints들이 P&R에 어떤 영향을 주는지를 파악해 두는 것도 유용하다.

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netWeight

The netWeight function sets the net priorities (or weights) for placing cells. During placement, the Milkyway-based tool attempts to place cells connected to high-priority nets closer together.

The primary goal of your Synopsys application, regardless of the specified net weights, is to achieve the minimum total net length for the design. Therefore, the Synopsys application weighs any reduction in the length of a high-priority net against the effect on the overall net length.

Nets with no priority set have the lowest priority (net weight of 1).

You can set net weights while running a Synopsys application by using Constraints > Set Net Weight.

Syntax

netWeight netName netWtHorizontal netWtVertical 

where the arguments are as follows:

netName
         Name of the net to be prioritized.

         Valid values: Name of any net in the design
netWtHorizontal
         Weight to be given to horizontal wires on the net. The higher the value, the greater priority the Synopsys application gives the net.

         Valid values: Any positive integer
netWtVertical
         Weight to be given to vertical wires on the net. The higher the value, the greater priority the Synopsys application gives the net.

         Valid values: Any positive integer

Example

netWeight “CTL2″ 10 8
netWeight “CTL3″ 4 2
netWeight “CTL4″ 6 6

Using these net priority directives, the Synopsys application gives the highest priority to horizontal wires on the CTL2 net and the lowest priority to vertical wires on the CTL3 net (except for nets not specified, which receive the lowest priority). For nets CTL2 and CTL3, horizontal
wires are given higher priority than vertical wires. For net CTL4, horizontal and vertical wires are given equal priority.

출처: Astro 2007.03-SP4 Help